This invention relates generally to the glass blowing arts and more specifically to an improved dual mixing gas burner for use in those arts.
For many years, the standard lathe ring burner used in the glass blowing industry has been of the types marketed by Litton Engineering Laboratories and by Carlisle Machine Works. Originally designed as semi-surface mix burners for natural gas and propane, these prior art burners have been used extensively for working the new higher temperature boro-silicate glasses. With the birth of the semiconductor industry in 1964 and the subsequent wide use of silica and clear fused quartz as important industrial glasses, the Litton burner has been used with hydrogen gas to obtain the extremely high working temperatures that these glasses require. The growth of semiconductor technology through successive generations of larger and larger silicon wafers brought about the requirement for larger quartz tubing diameters. With the development of good quality large diameter quartz tubing came the need for larger gas burners with which to work it. The typical response to this need was to make the standard ring burner larger. The typical glass shop work horse burner of 1970 was a ring burner with eight single jet heads or six seven jet heads, while now that burner has grown to one with twelve or fourteen seven jet heads and, even at that size, it is hard pressed to develop the heat densities required to comfortably work the larger diameter tubes.
These prior art Litton burner heads, available in both single and seven-jet configurations, are simple press-fit systems with side ports for the introduction of oxygen and fuel gas. The fuel gas entering one side port flows into an annular region and is encouraged to disperse to a uniform density by means of baffles before entering the open mixing area at the top of the burner. Oxygen, entering from the other side port, flows to the center core of the burner and thence via one or seven small orifices into the mixing area. The combined gases, when ignited, exit the burner at the top opening. The mixing region of this burner can be defined as the interior of the cylinder formed by the surface connecting the top opening and the circumference of the center core nose. The flux of fuel gas passing through this surface becomes the amount of fuel available for combustion. Similarly, the amount of oxygen available for combustion is the flux passing through the surfaces defined by the orifice(s) in the nose of the center core. The mixing that occurs in this burner may thus be understood as two gases flowing through each other at more or less cross directions. Ideally, the gases are ignited, a well defined flame front is established at each oxygen jet orifice, and the resulting venturi tends to entrain any uncombusted gas into the flame. In practice, however, the diffusion characteristics of the mixed gases vary significantly.
In the case of standard hydrocarbon fuel gases, the coefficient of diffusivity is much less than that of oxygen. At any given time, the mixing process can be viewed as oxygen flowing from the orifice(s) and diffusing from that flow into a static field of fuel gas. Such a process should be very efficient as each molecule of fuel gas in the definite volume of the mixing area has a good chance of being met by oxygen molecules diffusing from the various flow jets.
In the case of hydrogen as a fuel gas, the situation is reversed. The diffusivity of hydrogen is about three and one-half times greater than that of oxygen. Thus, at any given time, the mixing process can be understood as a flux of hydrogen diffusing into well defined oxygen jets. As a rough gauge of the efficiency of mixing, one can calculate the probability of any given molecule of hydrogen diffusing into an oxygen jet. This probability is estimated by the proportion of the mixing region's defining surface area and the surface area of the oxygen jets. In the case of the Litton seven-jet burner, this probability is 63%. In practice, the measured efficiency of hydrogen combustion in this burner is somewhat better, due to the fact that mixing occurs by cross flow as well as by diffusion, by the venturi entrainment of some uncombusted hydrogen, and by entrainment of the surrounding atmosphere. Measured efficiencies range from 65% to 80%, depending on the oxygen mix of the flame. Various glass blowing procedures require different types of flames. Soft flames are generally associated with low gas flows and are rich in fuel. Hard flames tend to require high gas flows and are generally lean. Optimum heat transfer is usually associated with high flows and stoichiometric or slightly rich gas proportions. The 80% mixing efficiency in the Litton burner head occurs at optimal heat transfer conditions. In any case, a visual inspection of the flame shows a marked hydrogen envelope of uncombusted fuel. Given the high diffusivity of hydrogen compared to oxygen, it is impossible, given the mixing design of the Litton prior art burner, to completely burn the available fuel.
In contrast, another prior art burner, of the type marketed by American Gas Furnace Company (AGF), is a true surface mix or gas diffusion flame burner specifically designed to burn hydrogen gas. These burners consume huge volumes of hydrogen and produce sufficient heat to easily work quartz in diameters up to fourteen inches. While these burners are excellent for smooth build up procedures, they do not produce flames of sufficient sharpness or delicacy for most of the more typical lathe operations. That fact, their high cost resulting from nickel alloy Inconel construction needed to withstand the high radiant heat produced by white hot quartz, and the special fixed installations they require have limited their use to operations involving large diameter quartz tubing.
Unlike the Litton burner head in which the fuel gas and oxygen are semi-mixed before leaving the burner proper, the AGF burner is a true surface mix or diffusion flame burner in which gas mixing occurs outside the burner by the gases diffusing off stream into each other. Rather than introducing oxygen jets into a field of fuel gas, the flame produced by the AGF burner is a composite of several (16 to 177) single jets bound in a simple case. The hydrogen fuel is introduced into the top chamber and is allowed to flow through a given number of orifices in the face plate. Oxygen is directed into the bottom chamber and thence into hypodermic tubing that transports it to the center of each hydrogen orifice. There, given the overwhelming diffusivity of hydrogen over oxygen, the hydrogen diffuses into the center oxygen jet and also away from the center. Upon ignition, the several flame jets coalesce into one well defined flame. Any hydrogen that diffuses away from its jet center is presumably caught up in the flame of a neighboring jet. While this holds for jets in the interior of the flame, those jets around the periphery of the flame will lose a substantial portion of their available fuel. Maximizing the fuel combustion efficiency of these burners is a matter of maximizing the number of interior gas jets relative to the number of those on the perimeter.
More recently, Weiss Scientific Glassblowing and G.M. Associates have marketed a burner head that is a combination of the Litton burner head and the American Gas Furnace Company surface mix burner. These burner heads, generally known in the industry as Litton Replacements and available in three or seven jet configurations, have the advantages of fitting the standard Litton and Carlisle ring burners, have an improved flame geometry, generate an intense heat density, and are relatively quiet. They are significantly disadvantageous in that they waste a substantial amount of gas and heat. The seven-jet version, for example, produces a flame not dissimilar to a miniature version of the flame produced by the American Gas Furnace Company burner that is too wide, too hard, and too diffuse for most work. The three-jet version, on the other hand, produces a flame that is acceptable for most work, but fails to combust as much as one-half of the hydrogen gas flowing through it. While these Litton Replacement burner heads represent a significant advance in providing flames of sufficient quality to work large diameter quartz tubing, the wasted fuel and misdirected heat accompanying this generally improved performance represents a serious impediment to their acceptance in the industry.
These Litton Replacement burner heads are essentially miniature AGF burners designed to be fitted into the standard Litton and Carlisle ring burners. Again, hydrogen is directed into one side port, conveyed upward into a chamber and thence out the given number of orifices in the face plate. Oxygen is introduced into the opposite side port, conveyed downward into a chamber, and thence into hypodermic tubing to be transported to the center of the hydrogen orifice.
As noted above in the discussion of the AGF burner, the efficiency of combustion of these burner heads is dependent upon the probability of hydrogen diffusing away from the center of its flame jet to diffuse into another neighboring flame jet. The combustion efficiency of these burners is expected to be poor since nearly all of the jets lie on the perimeter of the flame. For example, in the seven-jet model marketed by G.M. Associates, the percentage of hydrogen diffusing away from the center of the three main jets is 66%. The percentage of hydrogen diffusing away from the center of the four minor jets is 75%. If roughly two-thirds of that outward diffusing fuel is lost at the jets on the permiter and none is lost at the center jet, then the overall combustion efficiency of the burner is about 59%. It is worse in the case of G.M. Associates' three-jet model, where two-thirds is a reasonable loss for the center jet, with a corresponding loss of five-sixths for the two outside jets. This results in an overall combustion efficiency of about 49%. The foregoing probability estimates are based on the nearest neighbor exposure typical of hexagonal close pack spacing. Again, in practice the measured efficiencies tend to be somewhat better due to the venturi induced entrainment of uncombusted gases. Measured efficiencies for the G.M. Associates three and seven jet models of the Litton Replacement burner are 52% and 60%, respectively, measured at optimum flows for maximum heat transfer.
The basic problem common to all of the prior art burners discussed above is that their designs fail to take into account fuel diffusivity. If the standard hydrocarbon gases are used as fuels, this design consideration is of no consequence, since the fuel tends to remain in the stream into which it is initially directed, and the oxidizer diffuses into it. Indeed, diffusion loss of oxygen presents no problem, since there is an adequate replacement supply in the surrounding atmosphere in which these flames generally burn. In the case of hydrogen as a fuel, the design defect is much more apparent. Hydrogen is expensive, and the loss of any fuel in an application where every bit of available heat is keenly appreciated is simply counterproductive.
If the loss of hydrogen by diffusion away from the flame jet is the major obstacle to optimal performance, a possible solution is to trap the hydrogen so that it cannot escape. One way of doing this would be to reverse the hydrogen and oxygen flows through the burner, resulting in injecting hydrogen into oxygen rather than vice versa. Then, the hydrogen has nowhere to diffuse other than into the oxygen, thus providing complete combustion. A large hand burner known in the prior art as the Multimix Torch available from Wale Apparatus, Inc. is typically run backwards by most glassblowers. That is, hydrogen is applied to the oxygen inlet, and oxygen is applied to the hydrogen inlet. This technique is somewhat useful, given the particular geometry of the Multmix Torch, but it does not work at all with the prior art burners discussed above. The reason that hydrogen is not generally injected into oxygen is that fuel diffusion and flame dynamics then operate at cross purposes. The flame front develops where the hydrogen diffuses into the oxygen with outward momentum. The flame sets up a venturi action that tends to contract the flame front with inward momentum. The result is a diffuse, unfocused flame front, low heat density, and poor transfer of heat from the flame to a glass workpiece.
It is therefore a principal object of the present invention to provide an improved hydrogen gas burner in which dual mixing of the applied gases is employed to provide improved heat transfer from the flame to the glass workpiece and to provide more complete combustion of the hydrogen fuel gas, resulting in a significant cost saving of expensive hydrogen gas.
It is a further object of the present invention to provide an improved hydrogen gas burner having a rear gas inlet configuration.
These and other objects are accomplished in accordance with the illustrated preferred embodiments of the present invention by providing a gas burner comprising a cylindrical or other shaped housing having a flat front burner surface and a flat rear surface. A fuel gas chamber and two oxygen chambers are located within the housing. A central oxygen jet conveys oxygen gas from the first oxygen chamber to the front burner surface. A fuel gas jet, coaxially positioned with and surrounding the central oxygen jet, conveys fuel gas from the fuel gas chamber to an annular area on the front burner surface around the central oxygen jet. An outer oxygen jet, coaxially positioned with and surrounding the fuel gas jet, conveys oxygen gas from the second oxygen chamber to an annular area on the front burner surface around the fuel gas jet. Any number of the above combinations of a central oxygen jet, fuel gas jet, and outer oxygen jet may be provided to convey oxygen gas and fuel gas to the front burner surface. Ports for the entry of oxygen gas and fuel gas may be located on the cylindrical surface or on the flat rear surface of the gas burner.